How To Set Up Software RAID1 On A Running System (Incl. GRUB2 Configuration) (Ubuntu 10.04) - Page 4

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  1. 9 Testing
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9 Testing

Now let's simulate a hard drive failure. It doesn't matter if you select /dev/sda or /dev/sdb here. In this example I assume that /dev/sdb has failed.

To simulate the hard drive failure, you can either shut down the system and remove /dev/sdb from the system, or you (soft-)remove it like this:

mdadm --manage /dev/md0 --fail /dev/sdb1
mdadm --manage /dev/md1 --fail /dev/sdb2
mdadm --manage /dev/md2 --fail /dev/sdb3

mdadm --manage /dev/md0 --remove /dev/sdb1
mdadm --manage /dev/md1 --remove /dev/sdb2
mdadm --manage /dev/md2 --remove /dev/sdb3

Shut down the system:

shutdown -h now

Then put in a new /dev/sdb drive (if you simulate a failure of /dev/sda, you should now put /dev/sdb in /dev/sda's place and connect the new HDD as /dev/sdb!) and boot the system. It should still start without problems.

Now run

cat /proc/mdstat

and you should see that we have a degraded array:

[email protected]:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sda3[0]
      4242368 blocks [2/1] [U_]

md1 : active raid1 sda2[0]
      499648 blocks [2/1] [U_]

md0 : active raid1 sda1[0]
      498624 blocks [2/1] [U_]

unused devices: <none>
[email protected]:~#

The output of

fdisk -l

should look as follows:

[email protected]:~# fdisk -l

Disk /dev/sda: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000246b7

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          63      498688   fd  Linux raid autodetect
Partition 1 does not end on cylinder boundary.
/dev/sda2              63         125      499712   fd  Linux raid autodetect
Partition 2 does not end on cylinder boundary.
/dev/sda3             125         653     4242432   fd  Linux raid autodetect
Partition 3 does not end on cylinder boundary.

Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/sdb doesn't contain a valid partition table

Disk /dev/md0: 510 MB, 510590976 bytes
2 heads, 4 sectors/track, 124656 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/md0 doesn't contain a valid partition table

Disk /dev/md1: 511 MB, 511639552 bytes
2 heads, 4 sectors/track, 124912 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/md1 doesn't contain a valid partition table

Disk /dev/md2: 4344 MB, 4344184832 bytes
2 heads, 4 sectors/track, 1060592 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/md2 doesn't contain a valid partition table
[email protected]:~#

Now we copy the partition table of /dev/sda to /dev/sdb:

sfdisk -d /dev/sda | sfdisk --force /dev/sdb

[email protected]:~# sfdisk -d /dev/sda | sfdisk --force /dev/sdb
Checking that no-one is using this disk right now ...

Disk /dev/sdb: 652 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/sdb: unrecognized partition table type
Old situation:
No partitions found
New situation:
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/sdb1   *      2048    999423     997376  fd  Linux raid autodetect
/dev/sdb2        999424   1998847     999424  fd  Linux raid autodetect
/dev/sdb3       1998848  10483711    8484864  fd  Linux raid autodetect
/dev/sdb4             0         -          0   0  Empty
Warning: partition 1 does not end at a cylinder boundary
Successfully wrote the new partition table

Re-reading the partition table ...

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)
You have new mail in /var/mail/root
[email protected]:~#

Afterwards we remove any remains of a previous RAID array from /dev/sdb...

mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdb2
mdadm --zero-superblock /dev/sdb3

... and add /dev/sdb to the RAID array:

mdadm -a /dev/md0 /dev/sdb1
mdadm -a /dev/md1 /dev/sdb2
mdadm -a /dev/md2 /dev/sdb3

Now take a look at

cat /proc/mdstat

[email protected]:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[2] sda3[0]
      4242368 blocks [2/1] [U_]
      [===>.................]  recovery = 16.1% (683520/4242368) finish=0.6min speed=97645K/sec

md1 : active raid1 sdb2[2] sda2[0]
      499648 blocks [2/1] [U_]

md0 : active raid1 sdb1[1] sda1[0]
      498624 blocks [2/2] [UU]

unused devices: <none>
[email protected]:~#

Wait until the synchronization has finished:

[email protected]:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[1] sda3[0]
      4242368 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      499648 blocks [2/2] [UU]

md0 : active raid1 sdb1[1] sda1[0]
      498624 blocks [2/2] [UU]

unused devices: <none>
[email protected]:~#

Then install the bootloader on both HDDs:

grub-install /dev/sda
grub-install /dev/sdb

That's it. You've just replaced a failed hard drive in your RAID1 array.


Falko Timme

About Falko Timme

Falko Timme is an experienced Linux administrator and founder of Timme Hosting, a leading nginx business hosting company in Germany. He is one of the most active authors on HowtoForge since 2005 and one of the core developers of ISPConfig since 2000. He has also contributed to the O'Reilly book "Linux System Administration".

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By: chandpriyankara

This is a great tutorial on RAID....

 we are looking for implementing other raid systems as well



By: Anonymous

This tutorial work also for debian squeeze, only problem with grub, delete recordfail and replace set root='(md0)' with set root='(md/0)'

By: Alexandre Gambini

In my try of implementacion of raid, the better choice was chance /etc/default/grub in option and uncomment this line GRUB_DISABLE_LINUX_UUID=true, and grub work fine for me

Thanks for the Tutorial, is great job


Before failing a drive (testing) open a second terminal window to monitor mdstat. In that window run this command "watch cat /proc/mdstat", if it is rebuilding, you must let it finish or you might kill your project. You can also monitor, in real time, other actions like failing partitions, etc...

 A wonderful project, a wonderful way to learn linux. Thank you.

By: ecellingsworth

This tutorial assumes you are issuing commands as root. If instead you are issuing commands as a less privileged user by using sudo, remember that you need to issue a separate sudo for both sfdisk commands in the piped command. Else you will get a "permission denied" error.

sudo sfdisk -d /dev/sda | sudo sfdisk --force /dev/sdb

I used this tutorial months ago to get my raid array started. A drive failed and I returned to this page today to remember how to rebuild a new drive. Forgetting the sudo tripped me up for a while. Good tutorial. I'm glad I took the time to set up the raid array. It saved me this time.

By: MC

I replaced a failing /dev/sda, and i put the old put /dev/sdb in /dev/sda's place

But it doesn't restart, It simply displays GRUB on boot.

Before shutting it down I did install GRUB on /dev/sdb

 I had to put the failing drive back in but it will probably fail soon.

 any help? maybe i have to flag it as boootable of do something in the bios?


By: jlinkels

In this tutorial it is explained like "failing" a device is sufficient test to see if an array is still operational or bootable.

The operational issue is fine, the bootable is not.

If you made a mistake or forgot to install the boot sector on both drives, the array will boot with a mdadm "failed" device, but it will not boot when a drive is disconnected, defective or gone.

So I strongly recommend that you actually disconnect one drive and see if the system boots. Then after resyncing, disconnect the other disk and try booting. 

Although failing and removing a device in mdadm is a good way to see if RAID is operational and can handle a disk failure during operation, it doesn't tell whether you correctly installed the boot loader. Often disks fail after a power cycle (as all hardware does...) and you don't want just to see a blinking cursor.


By: Bogdan STORM

Thank you for putting all this information together for everyone.

Very helpful.